1. Field of the Invention
This invention relates to automatic window coverings, specifically to an adaptive window covering system which is responsive to the natural (daylight) component of the spectrum of the ambient interior illumination, but insensitive to the spectrum produced by gaseous-discharge (e.g., fluorescent) lamps.
2. Discussion of Prior Art
Adaptive Window Coverings for Energy Savings
An adaptive window covering is a device which automatically self-adjusts to regulate the light admitted by a window. Such a device is particularly useful when used in conjunction with an adaptive lighting system, which automatically brightens or dims a high-efficiency lamp (typically of the gaseous-discharge type) to maintain a desired level of interior illumination. Used together, such systems can save substantial amounts of electricity by taking advantage of available daylight to reduce the need for artificial illumination--while still maintaining the desired quality of interior illumination.
In such an application, the energy savings are roughly proportional to the ratio of natural illumination to artificial illumination. Therefore, it is desirable to have an adaptive window covering which automatically adjusts to admit as much diffuse daylight as possible, without causing excessive brightness or glare to the room occupants. Several adaptive window covering systems suitable for this purpose are known in the art, and these can be grouped into broad two categories: closed-loop systems and open-loop systems.
Closed-loop Systems
A closed-loop adaptive window-covering system includes shading means (e.g., venetian blind, electrochromic panel, or other window covering) to vary the amount of light admitted by the window, measuring means to measure the interior brightness, and control means to register the output of the measuring means and actuate the shading means. For example, prior-art, closed loop systems include:
the window screen disclosed in U.S. Pat. No. 3,294,152 to Kuijvenhoven (1966); PA1 the brightness-regulating venetian blind disclosed in U. S. Pat. No. 3,646,985 to Klann (1972); PA1 the shutter control system disclosed in U.S. Pat. No. 4,396,831 to Yamada et. al. (1983); and PA1 the shutter control system disclosed in U.S. Pat. No. 4,622,470 to Makino et. al. (1986). PA1 The systems can be "loosely coupled", in which each system conveys its status (e.g., via an electrical signal) to the other system. This enables the window covering system to "know" the setting of the lighting system, and the lighting system to "know" the setting of the window-covering system. This, in turn, provides each system with sufficient information to operate in a way which maximizes energy savings. For example, the window covering system might be assigned an operating protocol which prohibits reducing the level of daylight unless the lighting system is already at minimum brightness, while the lighting system might be assigned a protocol which prohibits increasing the level of artificial light unless the window covering is already fully open. PA1 Alternatively, the systems can be "tightly coupled", sharing a common control element. This enables very sophisticated operating protocols which can achieve a high degree of energy efficiency. Such tightly coupled systems are disclosed, for example, by Selkowitz et. al. ("Realizing the DSM Potential Of Integrated Envelope and Lighting Systems", October 1993, LBL Report No. 34731, Lawrence Berkeley Laboratory, Berkeley, Calif.), and in U.S. Pat. No. 5,237,169 to Grehant (1993). PA1 The coupling between the two systems substantially increases the net cost. This cost increase comes in the form of either increased installation labor (when hard-wired connections are used), or increased hardware costs (when wireless connections are used). PA1 Many adaptive lighting systems are already installed and in use in the U.S., but a substantial fraction of these lack the necessary hardware and software interfaces to couple to an adaptive window-covering system. Moreover, even among those adaptive lighting systems which do have the necessary interfaces, there is limited standardization in the interface design and operating protocol. Therefore, the coupling approach is disadvantageous in the context of retrofit applications, in which a new adaptive window-covering system is used in conjunction with an existing adaptive lighting system. PA1 In a binary control system, the setting of the window covering alternates between two states (typically open and closed) as a function of the exterior light level. Systems providing the well-known "open-at-dawn, close-at-dusk" capability fall into this category. For example, the systems disclosed in U.S. Pat. No. 4,644,990 to Webb Sr. (1987) and U.S. Pat. No. 5,142,133 to Kern (1992) provide such a capability, and such systems have been available commercially for many years. PA1 they are unable to operate to maximize the ratio of natural to artificial illumination; PA1 they require interconnection with the lighting system, increasing system cost; PA1 they require modification to the existing lighting system, precluding retrofit applications; or PA1 they provide brightness-regulating accuracy which is inadequate for some applications. PA1 is capable of maximizing the ratio of natural to artificial illumination by automatically regulating the quantity of admitted daylight; PA1 requires no connection to the lighting system; PA1 requires no modification to the lighting system; and PA1 provides the accuracy of a closed-loop system.
Such closed-loop systems are capable of accurately regulating the net interior illumination, but suffer from one major problem which limits their energy-savings performance. As previously stated, it is necessary to maximize the ratio of natural to artificial illumination in order to maximize the energy savings. However, in such a closed-loop system, the measuring means measures the net interior illumination, and is incapable of discriminating between the natural and artificial components of the illumination--so the system does not have sufficient information to maximize the natural-to-artificial illumination ratio.
As an illustration of this problem, consider the behavior of such a system in response to an increase in the interior illumination. Such a system will then attempt to reduce the quantity of admitted daylight, even though the optimum energy-saving response (assuming that the admitted daylight is not excessively bright) would instead be to maintain the quantity of admitted daylight, while allowing the adaptive lighting system to reduce the brightness of the electric lights.
Coupling Between Lighting and Window-covering Systems
Some prior-art closed-loop systems attempt to solve the aforementioned problem by coupling the lighting system with the window-covering system. This can be done in two ways:
However, while coupling between the lighting and window-covering systems can solve the aforementioned problem, this approach suffers from two serious limitations:
Tailored Orientation of Measuring Means
Other prior-art, closed-loop, adaptive window-covering systems attempt to solve the problem of maximizing the natural-to-artificial illumination ratio by orienting the measuring means to face toward the window and restricting its field-of-view to the window area. In this way, the measuring senses primarily the admitted daylight, rather than the illumination generated by the lighting system. Such an approach is shown, for example, in the aforementioned disclosures of Makino and Yamada. In principle, such a system can maximize the natural-to-artificial illumination ratio by always admitting the desired maximum quantity of daylight.
However, in practice, this approach has a major limitation: unless the sensor is located close to the occupants of the room, the brightness measured by the sensor will not accurately indicate the brightness of the daylight sensed by the room occupants. For example, if the sensor is located near the bottom of the window and an occupant is seated near the center of the room, the sensor output will be maximized for high solar elevation angles, while the brightness sensed by the occupant will be maximized for medium solar elevation angles. Thus, for good performance, the sensor must be located near the room occupants, but this creates two additional problems. First, the required wiring (or other means of interconnection) between the sensor and the shading means increases the net cost of the system (via increased installation labor costs in the case of a hard-wired connection, or increased hardware costs, in the case of a wireless connection). Second, locating the sensor near the room occupants makes it difficult to exclude light from artificial sources, so that the sensor cannot accurately measure just the daylight component of the interior illumination.
An approach potentially capable of mitigating the difficulties outlined in the preceding paragraph is disclosed in U.S. Pat. No. 4,273,999 to Pierpoint (1981). Pierpoint shows an adaptive lighting system which maintains a predetermined, desired ratio of background-to-task illuminance at one or more work locations. Pierpoint's approach requires information concerning the daylight component of the illuminance at each location, which--as in the case of the Makino and Yamada systems--is obtained by means of window-mounted daylight sensors. However, Pierpoint also discloses a calibration step, in which a set of correlation coefficients are calculated which relate the sensor output to the actual daylight illuminance at a desired point in the room (as measured by specialized illumination instrumentation). After calibration is complete, the specialized instrumentation is no longer required, and the correlation coefficients can then be used to accurately infer the daylight illuminance solely from the output of the sensors. Although Pierpoint shows this technique in the context of an adaptive lighting system, the skilled artisan will appreciate that a similar technique could be used in an adaptive window-covering system, potentially overcoming some of the aforementioned difficulties.
However, Pierpoint's approach suffers from two serious disadvantages. First, the data used to calculate the correlation coefficients must span the full range of expected variation in weather and the position of the sun, requiring a very lengthy calibration process (ostensibly extending over a full year, in order to capture the full range of variation in the solar position). Second, a large number of correlation coefficients are required to adequately sample the full range of expected variation in environmental conditions. Third, Pierpoint's approach requires sufficient data memory to store the coefficients--and sufficient computing power to process them--to obtain the desired daylight illuminance estimates, increasing the cost of the required hardware and reducing its commercial viability.
Tailored Response Times
Still other prior-art closed-loop systems attempt to solve the problem of maximizing the natural-to-artificial illumination ratio by tailoring the response times of the window-covering and lighting systems. For example, the window-covering system can be designed to respond more quickly than the lighting system to dropping light, but less quickly to rising light levels. Then, if the net interior illumination suddenly increases, there will be a delay before the adaptive window covering reduces the quantity of admitted daylight, while the adaptive lighting system quickly dims to reduce the total illumination to the desired level. Conversely, if the net interior illumination suddenly decreases, there will be a delay before the adaptive lighting system brightens, allowing the adaptive window covering sufficient time to increase the quantity of admitted daylight. Such an approach is suggested, for example, in U.S. Pat. No. 5,237,169 to Grehant. In principle, such a scheme can maximize the natural-to-artificial illumination ratio without any connections between the window covering and lighting systems. However, such an approach suffers from at least three limitations.
First, window coverings employing mechanical shading means (such as venetian blinds) have a response time exceeding several seconds. Therefore, to implement this scheme, the lighting system's response time to decreasing light levels must be made very long (e.g., ten seconds), in order to allow the window covering sufficient time to complete its adjustment first. This can cause periods of inadequate interior illumination whenever the total interior illumination decreases.
Second, substantial intervals of inadequate or excessive brightness can occur when either the window covering or the lighting system reaches its limit of adjustment. For example, when the window covering is fully open and the light level decreases, there will be a period of inadequate illumination due to the intentional delay in the response of the lighting system. Similarly, when the lighting system is fully dimmed and the light level increases, there will be a period of excessive illumination due to the intentional delay in the response of the window covering system.
Third, this approach is disadvantageous in retrofit applications (in which a new adaptive window-covering system is used with an existing adaptive lighting system), since few existing adaptive lighting systems are capable of adjustable response times (and even fewer are capable of providing different response times to rising and diminishing light levels).
Open-loop Systems
Some of the aforementioned disadvantages of closed-loop systems can be overcome through use of open-loop techniques. An open-loop control system is one in which the variable to be regulated (in this case, the brightness of the admitted daylight) is not measured directly, but inferred from other information.
Open-loop techniques are feasible for adaptive window coverings because the transfer functions of many types of window coverings are known in the art (the transfer function of a window covering is a mathematical expression which defines the relationship between the incident and admitted illumination).
In an open-loop adaptive window-covering system, the measuring means measures the exterior, rather than interior, illumination. The control means then adjusts the shading means to a position which is a function of the exterior illumination, the desired quantity of admitted daylight, and the known transfer function of the shading means. This allows the illumination sensor to be located on the outward-facing side of the shading means, substantially reducing the errors caused by inadvertent sensing of the illumination produced by the electric lighting system. Thus, the open-loop approach is capable of maximizing the ratio of natural-to-artificial illumination, without any need for connections between the window-covering and lighting systems.
Two forms of open-loop control are known in the art:
Since these binary systems typically alternate between open and closed settings, there is no need to accurately characterize the transfer function of the venetian blind. Moreover, such systems are relatively easy to implement. However, they provide no dynamic regulation whatsoever of the admitted daylight (unless other means are added for that purpose, as in the case of Kern's system), and are therefore incapable of maximizing the ratio of natural to artificial illumination while simultaneously shielding the occupants from excessive brightness and glare.
In a proportional control system, the setting of the window covering varies more-or-less smoothly as a function of the exterior brightness level. The system disclosed in U.S. Pat. No. 5,532,560 to Element et al. (1996) is such a system. Another system providing an open-loop, proportional-control system is disclosed in my U.S. Pat. No. 5,663,621 (1997). As I discuss in detail in that disclosure, such a system is, in principle, capable of maximizing the ratio of natural to artificial illumination while simultaneously shielding the occupants from excessive brightness and glare.
However, the accuracy of open-loop, proportional-control systems in maintaining the desired quantity of admitted daylight depends, in turn, on the accuracy of the assumed transfer function. For many types of window coverings (e.g., venetian blinds), the transfer function is extremely complex, and can be practically implemented only with certain simplifications which reduce its accuracy. As discussed in my aforementioned patent, the resulting loss of accuracy is not an issue in many applications, and open-loop regulation is often sufficient. However, other applications require a level of accuracy which can only be provided by closed-loop systems.